Lecture 30 35 Antenna and Antenna Arrays

EEEC433 Dr. Navneet Gupta 1

Antennas

Dr. Navneet Gupta

Lecture-30-35

http://www.google.co.in/imgres?imgurl=http://www.global-b2b-network.com/direct/dbimage/50161858/Antenna.jpg&imgrefurl=http://www.global-b2b-network.com/b2b/88/91/1343/155761/sell_antenna.html&h=360&w=360&sz=21&tbnid=98lzlSKD5bNCbM::&tbnh=121&tbnw=121&prev=/images%3Fq%3Dimage%2Bof%2B%2Bantenna&hl=en&usg=__wyRlmG5vQGAmbHXkhqbcK0US9uQ=&ei=uXPISdO3EpD67APqsfCsAg&sa=X&oi=image_result&resnum=2&ct=image&cd=1

http://images.google.co.in/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/3/3b/Cassegrain_antenna.jpg&imgrefurl=http://commons.wikimedia.org/wiki/File:Cassegrain_antenna.jpg&usg=__xEZ5TYmjbGFmZatuTKih0iDE5Rc=&h=1046&w=1396&sz=650&hl=en&start=15&um=1&tbnid=fwF5kWeSelYaVM:&tbnh=112&tbnw=150&prev=/images%3Fq%3Dimage%2Bof%2B%2Bantenna%26hl%3Den%26sa%3DX%26um%3D1


IntroductionTwo-way wireless transmission

Key component: Antenna

Cellular phone handset Base station Tower

Antenna:

as an interface

as a mode transformer

as an impedance transformer

http://en.wikipedia.org/wiki/File:BTS_Vodafone.jpg


Basic Concepts

• The accelerating charge radiate energy i.e

they throw energy in their surrounding medium

RADIATION

Radiation is a phenomenon related to time-varying currents.

IEEE Definition of an antennaAntenna is that part of a transmitting or

receiving system which is designed to

radiate or to receive EM waves.


Basic Concepts

• Case I: d << λ

• Case II: d ≈ λ

No net radiation.

Radiation

antenna

TEM Line

[Note: In electromagnetics: (physical closeness) measured in terms of λ. If d <<

λ radiation is negligible]

d ≈ λ



• Ability to transmit power in a preferred direction

• The angular distribution of the transmitted poweraround the antenna (in the case of transmittingantenna), or received power (in the case of receivingantenna) is known as the radiation pattern

Directional Characteristics



Point-to-point communication between two towers

Transmitter Receiver

EEEC433 Dr. Navneet Gupta

8

Parabolic Dish Antenna

Giant Metrewave Radio Telescope

(GMRT), Pune India

The ability of an antenna to concentrate

power in a narrow beam depends on the size

of the antenna in terms of wavelength.

http://en.wikipedia.org/wiki/File:Electromagnetic-Spectrum.png


FIELD PATTERN POWER PATTERN


max

max

,

,,

,

,,

P

PP

E

EE

n

n

Normalized Pattern

Radiation intensity (W Sr-1)

The half-power level occurs

at those angles θ and φ for

which E θ (θ, φ)n = 1/√2 =

0.707

Or

Pn (θ, φ) = 1/2 = 0.5



Antenna pattern in rectangular coordinates and decibel (logarithmic) scale.


Also known as FNBW



RADIATION PATTERN

Beam area (ΩA) Antenna

Gain/

Directivity (D)

or Directive

Gain (G)

Effective Aperture (Ae)




Beam Area



φ0HP

θ0HP

• Directivity: The directivity D of an antenna is the ratio of

the maximum radiation intensity to the average radiation

intensity.

HPo

HPo

HPHP

A

A

D

D

D

410004

1

4

4

For an isotropic antenna


• Power Gain

• Radiation Efficiency/Antenna Efficiency

OTHER ANTENNA PARAMETERS


Effective Aperture (or Effective area)



Example:

Antenna Equivalent Circuits


Total power produced by the generator:

Power delivered to the antenna terminals

Power lost in the generator’s internal

resistance RG.


Maximum Power Transfer condition:

EEEC433 Dr. Navneet Gupta20

The Potential Functions

• We may determine the forms of the waves radiated from

an antenna if we know the current distribution over the

surface of the antenna.

• We will use vector potential approach



How to determine the radiation pattern?

Antenna Analysis:

(a) Determination of current

distribution on the

antenna structure.

(b) Determine A from J

(c) Field due to this current

distribution in the space

surrounding the antenna

– Find H from A

– Find E from H

(f) Calculate the power

density P


Hertzian Dipole

Figure A Hertzian dipole carrying current I Io cos wt.

tjer

edlIA

rj

w

04

Retarded Potential

φθr

φθr

θA r rAA

θ

r

aθ ra ra

rH

AH

sin

0

ˆsinˆˆ

sin

11

1

2

0



Hertzian Dipole

2

)(0 1

4

sin

0

0

rr

je

dlIH

H

H

rtj

r

w

βr << 1: near field zone condition

βr >> 1: far field zone condition

1/r3 : electrostatic field

1/r2: induction field

1/r: radiation field

32

)(0 1

2

cos

r

j

re

dlIE rtj

r

w

32

)(0 1

4

sin

r

j

rr

je

dlIE rtj

w

w

,where


Hertzian Dipole

rrad aHdsHEP ˆ2

1.Re

2

1 2*

Radiation resistance

2

0

2

240 Idl

Prad

2

2

2

0

80

2

dlR

I

PR

rad

radrad



Field Pattern of Hertzian dipole

E-Plane

H-Plane


3-D field pattern of far field (Eθ and Hφ) from Hertzian dipole

Field Pattern of Hertzian dipole



Power Pattern of Hertzian dipole

Important Antenna parameters of Hertzian dipole ?

• Directive gain

• Directivity

• HPBW

• Effective area (or aperture)



Omnidirectional Antennas


Parabolic Reflector Antenna


Basic idea: replace mechanical pointing & beam

forming by electronic means


Antenna Arrays• An antenna array is a group of similar antennas arranged in various

configurations (straight line, circles, triangle and so on) with proper

amplitude and phase relations to give certain desired radiation

characteristics.



The Very Large Array, Socorro, New Mexico, USA.


Another view of VLAThe VLA is the world's

largest radio telescope

array, consisting of 27

radio antennas each 25

meters in diameter and

weighing some two

hundred and thirty-five

tonnes. They are set out

in a Y shaped railway

network allowing the

antennas to be moved.

Two of the arms are 21

km in length, the third is

19 km. Photo by: David Nunuk


Two isotropic Point Sources

• Consist of two identical radiating elements (antennas)

spaced a distance apart.

– Case-1 Equal Amplitude and Phase:

– E = E1e-jψ/2 + E2e

-jψ/2

– E = 2E0cos(ψ/2)

E1=E2=E0 (say)

Lecture-34



Field Pattern between E versus θ for d = λ/2

The pattern is bidirectional, figure of eight. BROADSIDE ARRAY



Case-2 Equal Amplitude and Opposite Phase:

– E = -E1e-jψ/2 + E2e

jψ/2

– E = 2jE0sin(ψ/2)

END-FIRE ARRAY



Linear Array of N Isotropic Point Sources

• Linear array is equivalent to N individual elements that areequally spaced along a straight line and are uniform so thateach element is fed with current of the same magnitude but ofprogressive phase shift.



Consider a 1-D array of identical antennas, equally spaced

==> uniform progressive phase shift from one end to another.

cosd

Phase difference between adjacent sources

If reference is shifted to the

centre of array, φ is

eliminated

2sin

2sin

0

N

EEt

NE

NEE

nt

t

max,

0max,

2sin

2sin

N

N

En Normalized array

pattern



Array factors for uniform linear arrays


Broadside Array• In this array the phase angle is such that it makes

maximum radiation perpendicular to the line of array.

• In BSA, sources are in phase i.e δ = 0 and ψ = 0 for

max. must be satisfied.

• Major lobe maxima occurs at 900 or 2700.

• Directions of minor lobe maxima:

Nd

nor

2

12cos 1

minmax,

• Directions of pattern minima (of

minor lobes):

Nd

nor

1

minmin, cos



End-Fire Array• In this array the phase angle is such that it makes maximum

radiation along the line of array.

• In EFA, ψ = 0 and δ = -βd.

• Principal Maxima occurs at 00 or 1800.

• Directions of pattern maxima (of minor lobes):

112

cos 1

minmax,Nd

nor

• Directions of pattern minima (of minor lobes):

Nd

nor

2sin2 1

minmin,

The HPBW of the

broadside array is less

than that of the end-fire

array but the directivity

of the end-fire array is

larger than the

broadside array.

Directivity of the EFA is about 4 times higher than of the BSA of the same length.



Design criterion

• Visible range = 2βd (reduce it)

• What is the maximum permissible inter-element

spacing for which there is no grating lobe?

Depends upon the choice of the direction of the main beam

Grating lobes are

identical to the main

beam but in

undesired direction

leads to the

reduction in power

efficiency

BUT

Lecture-35


θ

βd < 2π

d < λ


For an end fire array, on the other hand, main beam is along θmax = 0,

the visible range of ψ is: -2βd ≤ ψvisible ≤ 0

Radiation pattern and visible range of an end-fire array

2βd < 2π

d < λ/2

http://www.analyzemath.com/antenna_tutorials/antenna_arrays.html


Tapering of Arrays• The technique used to reduce the unwanted side lobes

from the radiation pattern of antenna is known as

tapering.

• Side lobes can be eliminated if

Current amplitudes follows coefficient of binomial series

• Make the excitation amplitudes in the elements in

the center portion of an array higher than those in

the end elements.

Pascal’s Triangle



BUT:

Elimination of side lobes take place at the expense of

directivity. HPBW of binomial array is more than that of

uniform array for the same length of the array.

Example: N =5 d = λ/2


EFA with Increased Directivity• Ordinary end-fire array produces a maximum field in the end-fire

direction, it does not give the maximum directivity.

• To improve the directivity, Hansen and Woodyard proposed that the

required phase shift between closely spaced elements of a very long

array should be

Nd

Nd

Hansen-Woodyard condition

for increased directivity

For maximum in θ = 0o

For maximum in θ = 180o

2sin

2sin

2sin

N

NEn

Array Pattern

(increased directivity)



Field Pattern of 10-

source increased

directivity array with

λ/4 spacing



Beam Steering

• By adjusting the phasing of an array, it is

possible to maximize it in other directions.

• It is also possible to steer a null to a specific

direction. This is useful for reducing or

eliminating interference to a receiving antenna.

• Example: Four isotropic source array with beam

steering

Lecture 30 35 Antenna and Antenna Arrays

Documents

Transcript of Lecture 30 35 Antenna and Antenna Arrays